...
if (my_intput < 0) {
warn("Input was negative (%f) but it must be strictly positive; using zero", my_input);
my_input = 0.0;
}

void vcroak(const char *pat, va_list *args);

A variadic version of croak. This is useful when you create your own variadic function. For example, if you want to create your own croak that cleans up memory for you, you could try something like this: